On Coulomb drag in double layer systems

We argue, for a wide class of systems including graphene, that in the low temperature, high density, large separation and strong screening limits the drag resistivity behaves as d^{-4}, where d is the separation between the two layers. The results are independent of the energy dispersion relation, the dependence on momentum of the transport time, and the wave function structure factors. We discuss how a correct treatment of the electron-electron interactions in an inhomogeneous dielectric background changes the theoretical analysis of the experimental drag results of Ref. [1]. We find that a quantitative understanding of the available experimental data [1] for drag in graphene is lacking.Comment: http://iopscience.iop.org/0953-8984/24/33/335602

Experimental Demonstration of Optimal Unambiguous State Discrimination

We present the first full demonstration of unambiguous state discrimination between non-orthogonal quantum states. Using a novel free space interferometer we have realised the optimum quantum measurement scheme for two non-orthogonal states of light, known as the Ivanovic-Dieks-Peres (IDP) measurement. We have for the first time gained access to all three possible outcomes of this measurement. All aspects of this generalised measurement scheme, including its superiority over a standard von Neumann measurement, have been demonstrated within 1.5% of the IDP predictions

Effect of Holstein phonons on the electronic properties of graphene

We obtain the self-energy of the electronic propagator due to the presence of Holstein polarons within the first Born approximation. This leads to a renormalization of the Fermi velocity of one percent. We further compute the optical conductivity of the system at the Dirac point and at finite doping within the Kubo-formula. We argue that the effects due to Holstein phonons are negligible and that the Boltzmann approach which does not include inter-band transition and can thus not treat optical phonons due to their high energy of $\hbar\omega_0\sim0.1-0.2$eV, remains valid.Comment: 13 pages, 4 figure

Relativistic Doppler effect in quantum communication

When an electromagnetic signal propagates in vacuo, a polarization detector cannot be rigorously perpendicular to the wave vector because of diffraction effects. The vacuum behaves as a noisy channel, even if the detectors are perfect. The ``noise'' can however be reduced and nearly cancelled by a relative motion of the observer toward the source. The standard definition of a reduced density matrix fails for photon polarization, because the transversality condition behaves like a superselection rule. We can however define an effective reduced density matrix which corresponds to a restricted class of positive operator-valued measures. There are no pure photon qubits, and no exactly orthogonal qubit states.Comment: 10 pages LaTe

Chaotic Evolution in Quantum Mechanics

A quantum system is described, whose wave function has a complexity which increases exponentially with time. Namely, for any fixed orthonormal basis, the number of components required for an accurate representation of the wave function increases exponentially.Comment: 8 pages (LaTeX 16 kB, followed by PostScript 2 kB for figure

Non-linear operations in quantum information theory

Quantum information theory is used to analize various non-linear operations on quantum states. The universal disentanglement machine is shown to be impossible, and partial (negative) results are obtained in the state-dependent case. The efficiency of the transformation of non-orthogonal states into orthogonal ones is discussed.Comment: 11 pages, LaTeX, 3 figures on separate page

Nonclassical correlation in a multipartite quantum system: two measures and evaluation

There is a commonly recognized paradigm in which a multipartite quantum system described by a density matrix having no product eigenbasis is considered to possess nonclassical correlation. Supporting this paradigm, we define two entropic measures of nonclassical correlation of a multipartite quantum system. One is defined as the minimum uncertainty about a joint system after we collect outcomes of particular local measurements. The other is defined by taking the maximum over all local systems about the minimum distance between a genuine set and a mimic set of eigenvalues of a reduced density matrix of a local system. The latter measure is based on an artificial game to create mimic eigenvalues of a reduced density matrix of a local system from eigenvalues of a density matrix of a global system. Numerical computation of these measures for several examples is performed.Comment: v1: 10 pages, 8 figures, IOPART, v2: introduction modified, figure 7 replaced, v3: 10 pages, 10 figures, RevTeX4, major revision with an additional measure introduced, title changed (previous title: Non-classical correlation in a multi-partite quantum system reconsidered), to appear in Phys. Rev.

Scattering by linear defects in graphene: a tight-binding approach

We develop an analytical scattering formalism for computing the transmittance through periodic defect lines within the tight-binding model of graphene. We first illustrate the method with a relatively simple case, the pentagon-only defect line. Afterwards, more complex defect lines are treated, namely the zz(558) and the zz(5757) ones. The formalism developed, only uses simple tight-binding concepts, reducing the problem to matrix manipulations which can be easily worked out by any computational algebraic calculator.JNBR was supported by Fundacao para a Ciencia e a Tecnologia (FCT) through Grant No. SFRH/BD/44456/2008. NMRP was supported by Fundos FEDER through the Programa Operacional Factores de Competitividade-COMPETE and by FCT under project no. PEst-C/FIS/UI0607/2011. NMRP acknowledges both the hospitality and the funding from the Graphene Research Centre at the National University of Singapore, where this work was completed

Non-full rank bound entangled states satisfying the range criterion

A systematic method for generating bound entangled states in any bipartite system, with ranks ranging from five to full rank, is presented. These states are constructed by mixing separable states with UPB (Unextendible Product Basis) - generated PPT bound entangled states. A subset of this class of PPT bound entangled states, having less than full rank, is shown to satisfy the range criterion [Phys. Lett. A, vol. 232 (1997) 333].Comment: 6 pages, Latex. Minor corrections and additions. More references added. Accepted for publication in Phys. Rev.